The Russian authorities have held talks with an armed group who are holding hundreds of people hostage at a school in North Ossetia.
Families are facing an agonising wait for news
The hostage takers want Russian troops to leave Chechnya and the release of Chechen rebels held in Ingushetia.
At least 200 people, many of them children, were seized on Wednesday at the school in Beslan in North Ossetia.
The UN Security Council - at a meeting on Wednesday - condemned the hostage takers "in the strongest terms".
The 15-member council demanded "the immediate and unconditional release of all hostages of the terrorist attack", at a session requested by Moscow.
A BBC correspondent in Beslan says that families there got little sleep overnight, waiting near the school and hoping for news of their children.
Doctors and emergency teams are on stand-by, and the streets surrounding the school have been sealed off by soldiers and police.
"Let them go, they have nothing to do with it, they are not to blame," said one woman. "Let us replace our children."
The attackers are believed to have laid trip wires, saying they will blow up the school if stormed by police.
"They have said that for every fighter wiped out, they will kill 50 children, and for every fighter wounded, 20," North Ossetia's Interior Minister Kazbek Dzantiev was quoted by Reuters as saying.
There is confusion over the exact number of hostages, but Mr Dzantiev said probably between 300 and 400 people were inside.
The authorities say they believe the children's condition to be satisfactory.
The school crisis came a day after a suspected suicide bombing in Moscow killed 10 people. Last week the mid-air explosions of two passenger planes left 89 dead.
The Russian authorities are blaming what they call international terrorism, with suspicion falling on Chechen separatists, who have been fighting for independence from Moscow for the past decade.
Chechen rebel leader Aslan Maskhadov has denied that his forces are involved in the siege.
Wednesday was the start of the school term for millions of children across Russia, and parents also attend what is traditionally a day of celebration.
Masked men and women, wearing bomb belts, burst into the school, whose pupils are aged seven to 18, at around 0930 local time (0530 GMT).
RECENT DEADLY ATTACKS
31 Aug 2004 - Suicide bomb kills 10 at north Moscow train station
24 Aug 2004 - Two planes crash after leaving same Moscow airport, killing 89
May 2004 - Chechen president killed in blast at stadium in Grozny
Feb 2004 - Bomb attack kills at least 39 people on Moscow underground
Dec 2003 - Female suicide bomber kills five near Red Square
Up to 50 children were reported to have escaped in the confusion.
But several people, all adults, were said to have died as a result of the attack.
Russian President Vladimir Putin broke off a working holiday to return to Moscow to deal with the crisis.
The US called the siege "callous", and President George W Bush offered "support in any form", according to the Kremlin.
Correspondents say the siege bears the hallmarks of Chechen rebels.
They have a history of taking hostages to draw international attention to their independence demands - the most notorious incident being the Moscow theatre siege in 2002, which led to the deaths of 129 hostages.